Fast TOC-less UTC Service through Loran-C/Eurofix

1. Fast TOC-less UTC Service through Loran-C/Eurofix Prof. dr. Durk van Willigen, Arthur Helwig, Wouter Pelgrum & Gerard Offermans Reelektronika b.v. / GAUSS Research Foundation ILA 31 – Washington DC, October 27-30, 2002

2. Time: Which One? • Time driven by atomic references: • BIPM (International Time Bureau) • TAI (International Atomic Time) • Loran = TAI – 10 seconds (fixed) • GPS = TAI – 19 seconds (fixed) • Time driven by rotation rate of the earth: • UTC, basically what the British want to see on their watch • Earth-driven UTC has a variable number of integer Leap Seconds offset to atomic-driven TAI, Loran or GPS • UTC = TAI – LS (Leap Seconds) • DTAI = TAI – UTC (Recommendation ITU-R TF.460-6) • On October 28th, 2002: DTAI = 32 seconds Durk van Willigen et al – Reelektronika / GRF

3. How to Bring Time to Users? • WWV, DCF77, HBG, … • Worldwide, 1-ms class accuracy (sky waves), very low-cost, robust • Sends UTC • GPS • Worldwide, sub 1-µs class accuracy, low cost, vulnerable • Sends SV-time • Receiver computes GPS time which can be corrected into UTC Durk van Willigen et al – Reelektronika / GRF

4. How to Bring Time to Users? • Loran-C • Not worldwide, sub 1-µs class accuracy, medium cost, robust • Sends no time info • User resolves Loran time from • A priori known date/time and position • Computed TOC time • Phase comparison with Loran-C pulse • Maximum allowable a priori time error depends on sophistication of receiver Durk van Willigen et al – Reelektronika / GRF

5. Time Transfer in 2 Steps • Synchronize user clock oscillator to source clock • Time offset between user and source is stable but is not equal to zero • Steer time offset between user and source to zero, either by: • Varying user clock frequency • Introducing time steps in PPS output • Do both (course and fine steering) to get fastest acquisition of time Durk van Willigen et al – Reelektronika / GRF

6. How to Express Quality of Clocks? • Frequency makes Time: • Pulse per second (PPS) error expressed in • MTIE – Maximum Time Interval Error • TDEV – Deviation of Time Interval Error • MTIE & TDEV specified by • ITU-T => G.811 specs • ANSI => Stratum-1 specs Durk van Willigen et al – Reelektronika / GRF

7. Frequency Requirements versus Oscillator Specs Basic diagram downloaded from the Internet “Telecom Synchronization with GPS” Hugo Frueauf, Zyfer Inc.April 2002hxf@zyfer.com Caution: Clock-LoranTransition Area Statum-1 is most demanding spec in telecommunication Loran-C upper error limit for DSP receivers and accurate ASF Durk van Willigen et al – Reelektronika / GRF

8. GPS Time Is Excellent But… • Time is essential to keep modern society going • GPS is attractive, accurate and low-cost precise timing source worldwide • GPS is not very robust to interference • If GPS fails then three possible backups: • Cesium clock: frequency only, unless set to UTC • Excellent stability/month, 50 k$ class • Rubidium clock: frequency only, unless set to UTC • Excellent stability/day; 1-2 k$ class • Loran-C: frequency and time Durk van Willigen et al – Reelektronika / GRF

9. TOC Concept • Start of 1st pulse of PCI for all Loran-C chains coincided with the PPS (TAI) time tick on 1 January 1958 at 00:00:00 hrs • PPS = Pulse Per Second • TOC = Time of Coincidence of start of Loran-C PCI and a 1 PPS time tick • TOC repeats every 500 … 1,000 seconds in the US, Russia and Asia • TOCrepeatsevery 5,000 – 10,000 seconds in Europe!! Durk van Willigen et al – Reelektronika / GRF

10. TOC Difficulties • Large Time to First Fix of UTC time • USA, Russia and Asia: GRI/10 s (8 – 16 min) • NELS: GRI/100 s (1.4 – 2.8 hr) • Time and Day at first fix must be known better than within one GRI (40-100 ms) for single chain receivers • Multi-chain tracking reduces TTFF and a-priori time requirement through wide-laning significantly • Does not provide Year and Day of year Durk van Willigen et al – Reelektronika / GRF

11. Eurofix Offers Simple Solution • Broadcast concept: At the next beep time is … • Eurofix: At the end of the Eurofix type 4 message date and time at the start of next Master pulse is … • Maximum update rate of Day/Time can be up to once per 30 GRI, i.e. 1.5 to 3 seconds • If user position and ASF accurately known, and high-quality receivers used, time error relative to Loran-C station time can meet or exceed Stratum-1 specifications Durk van Willigen et al – Reelektronika / GRF

12. Eurofix Type 4.1 Message of 30 GRI’s says Next pulse starts at [Year, Day, Time] 1 2 3 30 1 2 1 GRI Next Eurofix Message of 30 GRI’s TOC-less UTC Concept Master and Secondary stations broadcast Type 4.1 Eurofix Messages containing nominal Year, Date and Time at which next pulse of Master station starts Eurofix Message Type 4.2 contains Leap Seconds and Precise Time Offsets from nominal transmission time values. It does not broadcast hours, days and years User must add ED to Master station value to find Year, Date and Time at whichnext pulse of Secondary station starts Durk van Willigen et al – Reelektronika / GRF

13. Pulse Start Times • The start of all emitted Master pulses theoretically coincides with 10 µs time ticks in Europe, and with 100 µs time ticks elsewhere • So worldwide, start times of master pulse transmission can uniquely be expressed in 10 µs units • Nominal Emission Delays of secondary stations do normally not match this 10 µs time tick frame outside Europe • Outside Europe user can find start time of secondary stations by adding nominal ED to Master start time • If ED is SAM controlled, then SAM correction values must also be provided to user. This transfers SAM into TOE control if user has access to SAM correction data (RTCM PAPER 9-2001/SC104-246 by Bill Roland) • In Europe all secondary stations are TOE controlled, and ED’s are multiples of 100 µs. User needs no additional ED information Durk van Willigen et al – Reelektronika / GRF

14. Type 4.1 Message Format Durk van Willigen et al – Reelektronika / GRF

15. Type 4.2 Message Format Durk van Willigen et al – Reelektronika / GRF

16. Time Error Budget Total error = transmitter + propagation + receiver ≈100-150 ns ?? Megapulse, NELS, USCG ≈100 ns ?? Prof. Last, USCG, NELS ≈1-10 nsLocus, Reelektronika, Detectis,… Durk van Willigen et al – Reelektronika / GRF

17. TD-Observations of Sylt Sylt-Reeuwijk = 388 km Durk van Willigen et al – Reelektronika / GRF

18. Dual-rate Sylt TIE Time Interval 100 sec 1,000 sec 10,000 sec Durk van Willigen et al – Reelektronika / GRF

19. TDEV of dual-rate Sylt TIE Durk van Willigen et al – Reelektronika / GRF

20. Time Steps at Sylt Sylt-Reeuwijk = 388 km Durk van Willigen et al – Reelektronika / GRF

21. Discrete TX Time Control Optimal? 24-Hrs X-rate TD in Reeuwijk at 388 km from Sylt Single-station dual-rate offers canceling of ASF, diurnal, and receiver clock effects Reelektronika LORADD Receiver Durk van Willigen et al – Reelektronika / GRF

22. TDEV LPA-less Dual-rate Sylt TIE Time Interval 100 sec 1,000 sec 10,000 sec Durk van Willigen et al – Reelektronika / GRF

23. LPA-less Dual-rate Sylt TIE Durk van Willigen et al – Reelektronika / GRF

24. Emission Time Accuracy? • Europe (NELS) • TOE of all stations better than 100 ns (1) relative to UTC • ED better than 30 ns (1) • Outside Europe (USA, Russia, Asia, …) • Master stations better than < 100 ns relative to UTC (1) • Secondary stations better than < 140 ns relative to UTC (1) Durk van Willigen et al – Reelektronika / GRF

25. Loran-C/Chayka Station Tasks • Extract date and time from Loran time • Compute date/time at start of first pulse of Masterstation after end of next Eurofix type 4 message to be broadcast • Generate content of next Eurofix type 4 message to be broadcast by Master or Secondary station Durk van Willigen et al – Reelektronika / GRF

26. Low-End Timing Receiver Issues • Single-chip receiver design • Needs only one single Loran-C station • Environment dictates selection of E-field or H-field antenna • Simple Xtal clock performs coasting • Propagation time not anticipated • Accuracy better than 1 GRI (<100 ms) • Competition from other time sources • Europe: DCF77 clocks (Mainflingen / 77.5 kHz) • Elsewhere: ? Durk van Willigen et al – Reelektronika / GRF

27. High-End Timing Receiver Issues • Control of signal-delay from antenna to analog-to-digital converter • Skywave, CW and X-rate interference • Re-radiation errors • ASF model errors and unknown diurnal effects • SNR of received Loran-C/Chayka signal • Determines clock-to-Loran-C transition region • Tracking-loop bandwidth optimization • Determines clock-to-Loran-C transition region • Robust Stratum-1 performance with Rubidium or OCXO clock • Just about 5 % of Cesium clock price (≈50 k$) • GPS is the only true competitor in this class of accuracy Durk van Willigen et al – Reelektronika / GRF

28. Conclusions • NELS approved proposal to implement TOC-less UTC message transmissions on the four Eurofix stations at Bø, Værlandet, Sylt and Lessay • This service will offer robust and precise time (1 µs class) in Europe which can easily be expanded to the USA, Asia and Russia (Chayka) • Message type conform Recommendation ITU-R M.589-3 and RTCM SC-104, version 2.3 • Open signal structure is business invitation to industry Durk van Willigen et al – Reelektronika / GRF